Means for preventing the deleterious effects of x-rays in resonant cavity magnetrons



A. w. CROOK EI'AL 2,884,563

US EFFECTS OF MEANs FOR PREVENTING THE DELETERIO X-RAYS IN RESONANT CAVITY MAGNETRONS 1 April 28, 1959 Filed Nov. 18, 1957 INVENTORS] mow wmm M 7m F/uAmZuQ M v B7: fiaazw n ATTORNEY:

MEANS FOR PREVENTING THE DELETERIOUS EFFECTS OF X-RAYS IN RESONANT CAVITY MAGNETRONS Archie Watson Crook and Peter Frederick Lewis, Essex, England, asslgnors to English Electric Valve Company Limited, London, England, a company of Great Britain Application November 18, 1957, Serial No. 697,127

Claims priority, application Great Britain February 6, 1957 6 Claims. (Cl. 315-3953) This invention relates to resonant cavity magnetrons 1.e. to magnetrons of the kind in which an anode block, co-axial with a cathode, is provided with a plurality of resonant cavities formed in the block. It is customary in such magnetrons to take off generated high frequency energy through an output arrangement comprising a hole in the block leading to the back of one of the cavities. In some cases a loop in the cavity leads to a conductor passing through the hole. In others the hole forms part of a waveguide section coupled into the cavity. In either form of arrangement vacuum tightness is preserved by providing a sealing member of dielectric material across the hole and through which the conductor or waveguide passes.

Because of its high operating voltages, a magnetron is a source of X-rays and difliculties have been experienced because a proportion of X-rays are radiated into the output arrangement causing ionization of gas in said arrangement outside the vacuum envelope of the tube in some cases causing voltage break-down in the co-axial line or waveguide output. It is common practice to mitigate this defect by mounting the output arrangement at a suitable angle with respect to the center line of the cavity i.e. the radius from the magnetron cathode, so that X-rays do not pass into the co-axial line or waveguide. This practice however, results in the defect that the interior walls of the output arrangement are still subjected to bombardment by X-rays, and by this means photo-electrons are produced. These electrons, accelerated by fields in the output arrangement, bombard other surfaces of said arrangement causing the emission of secondary electrons which in turn are accelerated by said fields and cause further bombardment of said surfaces. The above effect, which is sometimes termed multipactoring is therefore cumulative and results in the generation of considerable heat and also in the loss of a substantial proportion of the output power of the magnetron. In extreme cases multipactoring may cause the output arrangement to become so hot that the dielectric sealing member melts thus destroying the vacuum in the tube. The objectof the present invention is to overcome these defects.

Experiment has now revealed that electron bombardment of the surfaces of the resonant cavities where they open into the inter-action space of the magnetron causes these surfaces to act as a major source of X-rays.

According to this invention a resonant cavity magnetron having a resonant cavity from which generated high frequency energy is taken off through an output arrangement comprising a hole leading from said cavity and sealed with a member of insulating material is so constructed and arranged as to prevent or substantially reduce bombardment of said output arrangement by X- rays produced by electron bombardment of the surfaces of the output resonant cavity where said cavity opens into the magnetron inter-action space.

A preferred way of carrying out the invention is so to choose the positional relation between the output arrangement and surfaces of the output resonant cavity where nited States Patent ice the latter opens into the inter-action space that practically the whole of said output arrangement is out of the paths of X-rays from said surfaces. Another way of carrying out the invention is to provide an X-ray screen to screen said output arrangement from X-rays. This alternative is, however, not preferred since it is not as simple, constructionally, as the first method.

The invention will be particularly described as applied to resonant cavity magnetrons of the so-called hole and slot type and of the rising sun type. In the first mentioned type each cavity is in the form of a cylindrical hole in the anode block and communicates with the interaction space of the magnetron through a slot. In the latter type the anode block has a plurality of radial tapered slots, alternately large and small, the narrower endsof which open into the interaction space of the magnetron. In the hole and slot type magnetrons as at present known the slots have parallel sides while in the rising sun type the sides of the slots diverge from one another in the direction away from the cathode. Experiment has revealed that the slot Walls constitute a relatively potent source of X-rays which then proceed across the cavity and bombard the output arrangement with the deleterious effects above described.

In applying the present invention to such magnetrons the corners of the junction of at least the output resonant cavity with the interaction space are cut away to leave surfaces which converge towards one another in a direc tion away from the cathode and by an angle of convergence such that no straight line drawn on either of said surfaces and continued impinges upon the output arrangement. If this is done X-rays generated by bombardment of the surfaces which act (as already described) as a major source of X-rays will not bombard any part of the output arrangement.

In another embodiment of the present invention which applies to magnetrons with the loop and coaxial line type of output arrangement the loop is itself made of tape or strip which is wide enough effectively to screen the cavity end of the hole of the output arrangement from X-rays generated at the walls of the slot.

The invention is illustrated in the accompanying drawings which show three different embodiments. In each figure the magnetron is shown only so far as is necessary to an understanding of the invention, part of the anode block only being shown in each case identified as follows:

Fig. 1 is a fragmentary plan view of the block of a hole and slot type of cavity magnetron partially broken away and shown in section illustrating the connection of the co-axial line to the output cavity of the magnetron and the means We provide for reducing the amount of X-rays radiated into the output arrangement;

Fig. 2 is a fragmentary plan view showing our invention applied to the anode block of a rising sun type of cavity magnetron; and

Fig. 3 is a fragmentary perspective view of a block of a hole and slot type of magnetron showing screening means applied to the structure for screening the output hole of the magnetron block from X-rays generated by bombardment of the side surfaces of the slot between the output cavity and the inter-action space.

Referring to Fig. 1 part of the block of a hole and slot type cavity magnetron is shown at 1, a plurality of resonant cavities 2, 3, 4, 5 and 6 being shown with slots 7, 8, 9, 10 and 11 opening out into the interaction space 12. The cavity 4 is the output cavity and generated high frequency energy is taken off by means of a loop 13 passing through a hole 14, and connected to the central conductor 15 of a co-axial line. Vacuum tightness is preserved by means of a sealing member 16 of dielectric material. It will be seen that the slot 9 is not (as is at present customary in hole and slot magnetrons) parallel sided over its whole length but is formed to have convergent side surfaces 17 such that lines (such as 18) drawn across these surfaces do not enter the hole 14 and therefore cannot impinge upon the sides 19 thereof.

As additional means for preventing X-rays from causing ionization of gases outside the vacuum envelope a metallic disc shaped screen 21 may be mounted upon the central conductor 15. Considerable modification of the output circuit may however be necessary to compensate for the etfect of said screen upon the electrical performance of the output arrangement.

Referring to Fig. 2 part of the anode block of a rising sun type of cavity magnetron is shown at 1'. A plurality of resonant slot cavities 2, 3', 5' and 6', alternately large and small, have mouths 7', 8', 9', 10 and 11' opening out into the interaction space 12'. The central cavity 4 is the output cavity and generated high frequency energy is taken off from it through hole 14' and waveguide 20. Vacuum tightness is preserved by means of a sealing member 16' of dielectric material. The output slot is not shaped (as are the other slots) as is at present customary in such magnetrons but has the corners at the mouth cut away to leave convergent side surfaces 17 such that lines (such as 18') drawn across these surfaces do not enter the hole 14 and therefore cannot impinge on the sides 19.

Fig. 3 shows part of the block of a hole and slot type of magnetron in which the output loop 22 is made of metal tape or strip wide enough effectively to screen the hole 14 from substantially all X-rays generated by bombardment of the side surfaces of the slot between the output cavity and the inter-action space. The walls of the slot may therefore be parallel sided as shown in Fig. 3 for the slots leading into the other cavities (2 and 3). As an additional safeguard, however, the slot 9 leading to the output cavity may be formed with convergent side surfaces 17 as in Figure 1 and this case is illustrated in Fig. 3.

We claim:

1. A resonant cavity magnetron including an output resonant cavity and an output arrangement for taking off generated high frequency energy from said output resonant cavity, said arrangement comprising a cavity having a hole leading from said cavity and sealed with a member of insulating material and an output lead from said cavity, a metallic disc-shaped screen, said screen being mounted upon the output lead and for rendering the amount of X-rays radiated into the output arrangement.

2. A resonant cavity magnetron of the hole and slot type wherein the corners of the junction of at least the output resonant cavity with the inter-action space are cut away to leave surfaces which converge towards one another in a direction away from the cathode and by an angle of convergence such that no straight line drawn on either of said surfaces and continued impinges upon the output arrangement.

3. A resonant cavity magnetron of the rising sun type wherein the corners of the junction of at least the output resonant cavity with the interaction space are cut away to leave surfaces which converge towards one another in a direction away from the cathode and by an angle of convergence such that no straight line drawn on either of said surfaces and continued impinges upon the output arrangement.

4. A resonant cavity magnetron including an output resonant cavity and an output arrangement having a hole and an output lead extending therethrough for taking off generated high frequency energy from said output resonant cavity, said output lead being of the loop and co-axial line type wherein the loop is made of metal tape sufficiently wide as effectively to screen the cavity end of the hole of the output arrangement from X-rays generated at the walls of the slot, said output arrangement having the hole therein extending from said cavity and sealed with a member of insulating material, and a metallic discshaped screen mounted upon said output lead for reducing the amount of X-rays radiated into the output arrangement.

5. A resonant cavity magnetron of the hole and slot type comprising a block having a multiplicity of slots and resonant cavities therein associated with an inter-action space and a cathode and including an output resonant cavity wherein the corners of the junction of at least the output resonant cavity with the inter-action space are cut away to leave walls having surfaces which converge towards one another in a direction away from the cathode and by an angle of convergence such that no straight line drawn on either of said surfaces and continued impinges upon the output arrangement, said output arrangement including a hole with a lead extending therethrough, said lead being of the loop and co-axial line type wherein the loop is made of metal strip sufliciently wide as to efiectively screen the cavity end of the hole of the output arrangement from X-rays generated at the walls of the associated slot.

6. A resonant cavity magnetron of the rising sun type comprising a block having a multiplicity of slots and resonant cavities therein associated with an inter-action space and a cathode and including an output resonant cavity wherein the corners of the junction of at least the output resonant cavity with the inter-action space are cut away to leave walls having surfaces which converge towards one another in a direction away from the cathode and by an angle of convergence such that no straight line drawn on either of said surfaces and continued impinges upon the output arrangement, said output arrangement including a hole with a lead extending therethrough, said lead being of the loop and co-axial line type wherein the loop is made of metal tape disposed in a position extending across the end of the hole in the output arrangement, said tape having a transverse width sufiiciently wide as to effectively screen the cavity end of the hole of the output arrangement from X-rays generated at the walls of the associated slot.

References Cited in the file of this patent UNITED STATES PATENTS 2,824,999 Walker Feb. 25, 1958 

